Asphalt composition having reduced ductility loss



United States Patent Office 3,133,823 ASPHALT COMPOSITIQN HAVING REDUCED DUCTILITY LOSS Harley F. Hardman, Lyndhurst, Ohio, assignor to The sarzilard Oil Company, Cleveland, Ohio, a corporation :o

No Drawing. Filed May 31, 1961, Ser. No. 113,603 2 Claims. (Cl. 106-230) This invention relates to an improved asphalt cement for paving purposes. More particularly, this invention relates to a method of improving the thermal stability of an asphalt cement to retard the loss of ductile characteristics which normally occurs during high temperature mixing with aggregate in a mix mill so as to improve the service behavior of an asphaltic pavement in which said asphalt cement is incorporated.

Good ductility is recognized by those skilled in the art as an important and desirable property for an asphalt cement intended for paving purposes. It is believed that the ductility of an asphalt cement is related to the actual service behavior of an asphaltic pavement in which it is incorporated so that as a qualitative matter the better the ductile characteristics of the asphalt cement present in the asphaltic pavement, the better will be the service behavior of the pavement.

In a paper entitled Cracking Characteristics of Asphalt Cement, by P. C. Doyle which appears in Volume 27 of the Proceedings of the Association of Asphalt Paving Technologists, at page 581, the author proposes that the ductile characteristics of asphalt cement are meaningful with respect to the tendency of an asphaltic pavement in which it is incorporated to fracture or crack in service, particularly if the ductile characteristics of the asphalt are measured on a ductilometer at lower temperatures and speeds than those commonly employed. The author reports tests on core samples which were drilled from pavements actually in service, showing both good and bad resistance to surface cracking. It was found from these various tests that when the ductile characteristics of the asphalt cement recovered from these core samples were run at 55 F. and 1 cm./min. speeds on the ductilometer the ductility values of the asphalt cement could be closely correlated with the service behavior of the pavement. By comparison, it was found that when the ductility of the recovered asphalt was determined at temperatures of 77 F. and at speeds of 5 cm./min. the ductility values obtained were not always consistent with field results with respect to the cracking tendency of the pavement. Besides showing that ductility measurements of the asphalt cement at 55 F. and 1 cm./min. speed on the ductilometer offers a convenient means for predicting the service behavior of an asphaltic pavement with respect to surface cracking, these results reported for these low temperature ductility measurements stress the importance of preserving good ductile characteristics in the asphalt in order to insure that the asphaltic pavement will provide good service behavior.

It has been found that asphalt cements generally have initially good ductile characteristics so that if this high degree of ductility Were possessed by the asphalt after being laid into a pavement, good service behavior of the pavement could be expected. However, it is well known that the ductility of the asphalt cement is materially lost during the high temperature mixing of the asphalt cement with aggregate so that the ductile characeristics of the asphalt cement in the pavement is usually of a relatively low value. For example, a typical asphalt for paving having a penetration value of 70 to 85 measured at 77 F.

3,133,823 Patented May 19, 1964 obtained from a Mississippi-Arkansas-Illinois crude had a ductility of 150+ centimeters (cms.) when measured on a ductilometer at 55 F., 1 cm. speed. This asphalt, however, after being mixed with gravel aggregate in a standard mix mill at 310 F. in accordance with conventional trade practices showed a ductility value of only 10 cms. when measured at 55 F. and 1 cm. speed. This, of course, demonstrates the drastic effect the high temperature mixing has on the ductile characteristics of an asphalt cement. There is, of course, some progressive hardening and loss of ductility in the laying of the asphalt and in its exposure to the elements while present in the pavement, but this loss is negligible in relation to the loss which occurs during the mixing operation. Consequently, if some way is found to inhibit the loss occurring during the mixing operation, the ductile characteristics of the asphalt can be largely preserved and the pavement in which it is incorporated will have greater resistance to cracking.

Generally, the degree of ductility loss in the mixing operation increases with the increase of temperature and/ or time employed in mixing and the time the finished mixture is held in a truck at the mixing temperature prior to its actual laying. According to good trade practices, the temperature at which a mix mill is operated is generally selected as the temperature which will give seconds viscosity on a Saybolt-Furol viscometer for the particular asphalt that is used. It is believed that this temperature provides the best film thickness of asphalt on the aggregate for paving purposes. However, since few mix mills are equipped with temperature indicating and/ or controlling devices, the operating temperature in actual practice of these mills has a tendency to vary over wide limits and the problem of the loss of ductile characteristics in the asphalt cement becomes particularly aggravated. Also, since the location of these hot mix paving plants are frequently distant from the paving job, it often happens during cold climatic conditions that the asphalt must be mixed at much higher temperatures than the proper mixing temperature in order that the paving mix will arrive at the job at a sutficient temperature for paving. Moreover, frequently the time of mixing is not carefully or uniformly controlled at the mix mill with commensurate depreciation of the ductile characteristics of the asphalt cement. In addition to these operational variables which may unduly deteriorate the ductile value, asphalt cements vary in their sensitivity to change in the high temperature mixing operation. Some asphalt cements therefore are much more sensitive than others depending on the crude oil from which they are produced and the method of manufacture. It will be obvious that the service life of many asphalt cements which have high heat susceptibility is drastically reduced by the mixing operation when the mixing is carried out at excessive temperatures or at proper mixing temperatures but for excessive mixing periods.

The primary object of the present invention therefore is to minimize the loss of ductile characteristics in an asphalt cement which occurs during the high temperature mixing with aggragate so as to improve the asphalts resistance to cracking in the pavement. It is a further purpose of this invention to permit much greater latitude in the time and temperature of mixing of asphalt with aggre gate. A still further object of this invention is to permit asphalt cements which are originally borderline with respect to ductile characteristics to be used in paving operations where they formerly could not be employed.

These objects and other objects which will become apparent from the following discussion may be accomplished in accordance with the present invention by add- 3 ing to the asphalt cement before the mixing operation one of the following chemical agents:

AGENTS (1) Zinc dibutyldithiocarbamate (2) Dilauryl selenide (3) P-nitroanisole (4) A mixture of 25 wt. percent zinc 2-ethyl hexoate and 75 wt. percent hydrogenated terphenols While each of the above agents taken individually provides a significant improvement in ductility characteristics, it has surprisingly been found that certain of these agents in combination with each other or with tri(2-ethylhexane diol-l,3)diborate interact to produce an unexpected, synergystic improvement. These combinations include the following:

(5) Dilauryl selenide +tri(2-ethylhexanediol-1,3 diborate (6) p-Nitroanisole-l-tri (Z-ethylhexanediol- 1 ,3 diborate (7) Dilauryl selenide+agent 4 (above) .(8) p-Nitroanisole+agent 4 (above) For some reason not fully understood the combination of tri(2-ethylhexanediol-1,3)diborate and agent 4 did not synergize.

In order to gain any significant effect, at least 0.001% by weight based on the weight of the asphalt cement of one or a mixture of the foregoing chemical agents must be used. Actually, there is no upper limit but amounts over 2% by weight usually cannot be justified economically. A preferred range for the chemical agent or agents is generally from 0.01% to 0.5% by weight. Where synergistic combinations are employed, an upgraded improvement may be realized where the weight ratio of individual agents in a given combination falls within the range of 0.25 to 1.0 and 4.0 to 1.0.

The chemical agent may be introduced to the asphalt cement at some convenient point before the asphalt is charged to the mix mill employing any suitable mixing means which will insure that the additive is evenly distributed throughout the asphalt. It is very desirable to add the chemical agent into the asphalt at the refinery while it is handled at elevated temperatures prior to loading for delivery. This may be accomplished in a heated tank utilizing a high-speed agitator as a mixing means.

The asphalt ingredient of this invention may be any natural or manufactured bituminous material which may be mixed with any of the common aggregates, such as crushed limestone, slag, crushed rock, sand, gravel, etc. to form an asphaltic concrete for paving. The desirable properties for such an asphalt cement may be found in highway specification manuals or in Abrahams text Asphalt and Allied Substances. In general, asphalt cement for paving purposes is required to meet penetration specifications and the preferred penetration will usually be from 50-200 at 77 F.

In preparing an asphaltic concrete for paving, the asphalt cement of the invention is mixed with aggregate in the proportion of from 4 to 8 parts by weight of asphalt to 96 to 92 parts by weight of aggregate at a temperature in the range of from 280 F. to 350 F., but preferably the temperature will be selected, as indicated before, to give 120 seconds viscosity (Saybolt-Furol) for the asphalt cement employed.

A better understanding of the present invention will be gained from the following discussion of oven weather tests comparing the loss of ductile characteristics during mixing with aggregate of an untreated asphalt cement with the same asphalt treated with the chemical agents of this invention.

The oven weather test stimulates the high temperature mixing in a conventional mix mill and correlates therewith. Results from this test have indicated that ductile characteristics measured on a sample of asphalt cement recovered from the hot paving mix prepared and heated in accordance with this test at 55 F., 1 cm. speed, parallel closely the ductile characteristics when measured at 55 F. 1 cm. speed, for a sample of the same asphalt cement recovered from a hot paving mix just before it is laid as a pavement following mixing in a conventional mix mill at the proper temperature to give 120 seconds viscosity (Saybolt-Furol) for the asphalt employed and including an average of approximately one hour transportation of the hot mixture to the job site.

In executing the oven test, 2000 grams of standard sand (ASTM, C-190, 20 to 30 mesh particle size) were placed in a bowl. Approximately 160' grams of asphalt cement were placed in a separate container. The sand and asphalt were thus heated separately for two minutes at the temperature which gives 120 seconds viscosity for the particular asphalt used, as determined by means of a Saybolt-Furol viscometer. Exactly grams of asphalt were added to the sand with mixing for two minutes.

900 grams of the asphalt-sand mixture was weighed into each of two circular aluminum pans measuring 12 inches inside diameter and inch deep. The mixture in each pan was smoothed out to provide uniform coverage in area and in depth. The pans and their contents were allowed to cool to room temperature.

The pans containing the asphalt mixture were then placed in an oven for exactly one hour, again at the temperature which gives 120 seconds viscosity for the asphalt being tested. The pans were then removed from the oven and allowed to cool to room temperature for at least one hour. The asphaltic mixture was scraped from the pans, placed in a Dulin Rotarex, and the asphalt was recovered by extraction with benzene in accordance with Absons method and test which is recorded in detail beginning at page 48 of the Association of Asphalt Paving Technologists Proceedings for 1952, vol. 21. The original asphalt and the recovered asphalt on evaporation of the benzene were tested for ductility according to ASTM designation D113-44 at 55:09" P. and at a rate of speed of 1 cm./min. The result reported for this test is the number of centimeters the sample specimen will extend before pulling apart.

A first series of runs using the oven weather test was conducted to illustrate the effect of the use of a small amount of individual chemical agents of the invention in reducing the loss in ductile characteristics in asphalt. The asphalt used was derived from a typical Mid- Continent crude having a penetration of 100, a ductility of at 77 F. and a second Saybolt-Furol viscosity at 294 F. The chemical agent for each run was added to the asphalt cement while the asphalt was maintained in a molten and mobile condition. A mechanical stirrer was provided to insure that the chemical agent was evenly dispersed throughout the asphalt. The results of this series of runs is reported in Table A below:

Table A Recovered Duct- Percent Improvement Cone, Additive Wt Duetility percent Decrease None Zn dibutyl-dithi ocarbamate. dilauryl selenide p-n itroanisole 25 wt. percent zinc 2 thyl hexoate 75 wt. percent hydrogenated terphenols 3 Q.- T able B Cone, Re- Percent Additlve Wt. covered Ductility Improvepercent ]53g%c% Decrease ment i ?Fa-trans"r 65 .iauryseenie n -e y- 3 hexartlediol-li3) iditbogte fi. 1 0.25 81 19 ms p-ni roaniso e ri et y 0.25

hexanedio1-1,3)diborate 0.25 i 73 27 4. dilauryl se1enide+Agent4 75 25 61.5 5. p-nitroanisole-I-Agent4 86 14 78.4 6. Agent 4 +tri(2-ethy1hexane- 0. 25

dio1-1,3)diborate 0.25 i

l A mixture of 25 wt. percent zinc Z-ethyl hexoate and wt. percent hydrogenated terphenols.

It can be seen from Table B that While tri(2-ethylhexanediol-1,3)diborate upgraded the improvement secured by the independent use of dilauryl selenide and p-nitroanisole, the same diborate had substantially no effect on the improvement secured by the independent use of the mixture of zinc 2-ethyl hexoate and hydrogenated terphenols.

It is to be understood that various modifications of the foregoing invention will occur to those skilled in the art upon reading the above description. All such modifications are intended to be included as may be reasonably covered by the appended claims.

I claim:

1. A material for paving exhibiting improved resistance to the loss of ductility consisting essentially of an asphalt cement and from 0.001% to 2% by Weight of an additive selected from the group consisting of (a) zinc dibutyldithio carbarnate; (b) dilauryl selenide; (c) p-nitroanisole; (d) a mixture consisting essentially of about 25 wt. percent zinc Z-ethyl hexoate and 75 Wt. percent hydrogenated terphenols; (e) a mixture consisting essentially of dialuryl selenide and tri(2-ethylhexanediol-1,3)- diborate in a Weight ratio within the range of 0.25 :T to 4.0:1.0; (f) a mixture consisting essentially of p-nitroanisole and tri(2-ethylhexanediol-1,3)diborate in a Weight ratio within the range of 0.25:1 to 4.0210; (g) a composition consisting essentially of (l) dilauryl selenide and (2) a mixture of about 25 Wt. percent zinc Z-ethyl hexoate and 75 wt. percent hydrogenated terphenols in a weight ratio Within the range of 0.25:1 to 4011.0; and (h) a composition consisting essentially of (1) p-nitroanisole and (2) a mixture of about 25 Wt. percent zinc 2-ethyl hexoate and 75 Wt. percent hydrogenated terphenols in a weight ratio within the range of 0.25 :1.0 to 4.0:l.0.

2. An asphalt concrete consisting essentially of a material defined in claim 1 and an aggregate.

No references cited. 

1. A MATERIAL FOR PAVING EXHIBITING IMPROVED RESISTANCE TO THE LOSS OF DUCTILITY CONSISTING ESSENTIALLY OF AN ASPHALT CEMENT AND FROM 0.001% TO 2% BY WEIGHT OF AN ADDITIVE SELECTED FROM THE GROUP CONSISTING OF (A) ZINC DIBUTYLDITHIO CARBAMATE; (B) DILAURYL SELENIDE; (C) P-NITROANISOLE; (D) A MIXTURE CONSISTING ESSENTIALLY OF ABOUT 25 WT. PERCENT ZINC 2-ETHYL HEXOATE AND 75 WT. PERCENT HYDROGENATED TERPHENOLS; (E) A MIXTURE CONSISTING ESSENTIALLY OF DIALURYL SELENIDE AND TRI(2-ETHYLEXANEDIOL-1,3)DIBORATE IN A WEIGHT RATIO WITHIN THE RANGE OF 0.25:1 TO 4.0:1.0; (F) A MIXTURE CONSISTING ESSENTIALLY OF P-NITROANISOLE AND TRI(2-EHTYLHEXANEDIOL-1,3)DIBORATE IN A WEIGHT RATIO WITHIN THE RANGE OF 0.25:1 TO 4.0:1.0; (G) A COMPOSITION CONSISTING ESSENTIALLY OF (1) DILAURYL SELENIDE AND (2) A MIXTURE OF ABOUT 25 WT. PERCENT ZINC 2-ETHYL HEXOATE AND 75 WT PERCENT HYDROGENATED TERPHENOLS IN A WEIGHT RATIO WITHIN THE RANGE OF 0.25:1 TO 4.0:1.0; AND ANISOLE AND (2) A MIXTURE OF ABOUT 25 WT. PERCENT ZINC 2-ETHYL HEXOATE AND 75 WT. PERCENT HYDROGENATED TERPHENOLS IN A WEIGHT RATIO WITHIN THE RANGE OF 0.25:1.0 TO 4.0:1.0. 